test: cover thrown errors from exec() kill

This commit adds code coverage for the scenario where exec()
kills a child process, but the call to ChildProcess#kill()
throws an exception.

PR-URL: https://github.com/nodejs/node/pull/11038


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>